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Assembly-Oriented Design

Assembly-Oriented Design. Dan Whitney April 5, 2002. Poll. We design assemblies explicitly as part of our product development process Our suppliers design our assemblies We design things and our manufacturing engineers try to get us to change them

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Assembly-Oriented Design

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  1. Assembly-Oriented Design Dan Whitney April 5, 2002

  2. Poll • We design assemblies explicitly as part of our product development process • Our suppliers design our assemblies • We design things and our manufacturing engineers try to get us to change them • We design parts using the best CAD system in the world and then we wonder why we have trouble assembling them • We don’t have any assembly problems

  3. Scope of “Assembly” • Assembly spans the entire range from point processes to business strategy • Assemblies are things that do something • Attributes • Architecture • Families, platforms… • Assembly is a process of putting things together • On the factory floor • Operations • Equipment • Ergonomics

  4. Scope of Assembly - 2 • Design for assembly • Part handling and mating • Part consolidation • Integral architecture favors performance • Modular architecture favors business issues • Design of assemblies - technical and business issues • Design intent • CAD representation • Key Characteristics • Math models, constraint, tolerances • Architectures, families, delayed commitment, flexibility

  5. Sony Does DFA During Concept Design

  6. Things an Assembly Theory Must Do • Represent top-level goals for the assembly • Link these goals to requirements on the assembly and the parts • Represent nominal and variedlocation of parts in space • Provide for declaration of mutual constraint between parts • Merge design of assembly and of assemblyprocesses, including adjustments and fixtures • Support a design process for assemblies that can be added to CAD

  7. This Theory of Assembly... • Focuses on “Kinematic Assemblies” • Emphasizes Delivery of Key Characteristics (KCs) • Documents KC Delivery and Constraint with the Datum Flow Chain (DFC) • Achieves Constraint with Assembly Features • Achieves DFC Robustness via Tolerance Analysis • Exploits Underconstrained Assemblies to Achieve Adjustments

  8. What Happens During Assembly • People think assembly is fastening • Assembly is really the chaining together of coordinate frames • These chains of frames “deliver” certain parts or features on parts to desired places in space relative to other parts or features on them within tolerances • Complete chains describe Key Characteristics of the assembly • This theory of assembly generates a design process for assemblies based on creating these chains

  9. “Chain of Delivery” of Quality No single part “delivers” the KC.

  10. Maintaining Oversight on KCs • To design the chains that deliver the KCs, we have developed the Datum Flow Chain (DFC) • A DFC is an assembly-level statement of design intent that- • documents the chain that delivers the KC • identifies the parts that make up the chain • provides a skeleton for the strategy by which the parts will be located in space as links in the chain • Each step in the assembly process adds links to the chain and each subassembly is kinematically constrained

  11. Office Stapler Liaison Diagram

  12. Datum Flow Chains in the Stapler The datum flow chain is a chain of constraining mates from one end of the KC to the other.

  13. Mates, Contacts, and KC Delivery Contact Mate Mates give location. Contacts reinforce location. Variation travels from part to part along the chain of mates.

  14. Coordinate Frames

  15. Chains of Frames = Assembly

  16. Datum Flow Chain for Car Front End F1 L. Body Side R. Body side 6 6 Underbody R. Door L. Door 6 F F 6 F Dash 6 6 6 R. O. Rail L. O. Rail L. I. Rail 6 R.I. Rail y,z F F x, z, x, y F x, z x, y F x, x,y, z x, x,y, z R. Apron F F L. Apron F x, y, z Hood fixture y, z, y, z x, y, z x, x y y, z, y, z x, x R. O. Shot. R. I. Shot. z, x y F y L. O. Shot. F F x, y, z z, x y L. I. Shot. z, y Hood F 6 6 x, y, z z, y L. Hinge R. Hinge R. Fender L. Bracket L. Bracket L. Fender Hood Latch x y, z x y, z y y Bolster R. Lamp L. Lamp x, z, x, y, z x, z, x, y, z Fascia Drawn by Gennadiy Goldenshteyn, MIT Student

  17. DFC for Aircraft Circumference

  18. DFC Carries Design Intent • Designer declares how KCs will be delivered • Intent is expressible in CAD terms • Intent expressed this way is independent of CAD vendor • DFCs can be bookshelved for future use

  19. T AF T FB A B T AF T FB A T AB B Connective Assembly Model Parts A and B are joined by two features The nominal location of part B can be calculated from the nominal location of part A using 4x4 transform math

  20. Varied Part Location Due to Variation The varied location of Part B can be calculated from the nominal location of Part A. This process can be chained to Part C, etc., including errors on Part B. It uses the same math as the nominal model.

  21. Stapler Variations

  22. When Parts are Joined, Degrees of Freedom are Fixed • Parts join at places called assembly features • Different features constrain different numbers and kinds of degrees of freedom of the respective parts (symmetrically) • Parts may join by • one pair of features • multiple features • several parts working together, each with its own features • When parts mate to fixtures, dofs are constrained

  23. Overconstrained and “Properly” Constrained Assemblies • Assemblies that function by geometric compatibility and force/moment equilibrium are called • statically determinate • “properly” constrained • “kinematic” or “semi-kinematic” • Assemblies that require the other principle of statics (stress-strain relations) are called • statically indeterminate • “over-constrained” • Constraint is a property of the nominal design

  24. Summary of Assembly Theory - Nominal Design • An assembly is a set of parts that deliver their quality, defined by the KCs, as a result of the geometric relationships between the parts (and fixtures) • Designing an assembly means designing these relationships in terms of one DFC per KC • The DFC documents the nominal relationships in terms of constraint • The DFC passes from part to part via mates • The nominal design is a constraint structure • Assembly features create the constraint relationships at each mate

  25. Summary of Assembly Theory - Variation Design • Tolerances should assure the robustness of the DFC • KC delivery is verified by a tolerance analysis of each DFC • Tolerances on parts derive from tolerances on the KCs • Part tolerances are sublinks of the DFC • Type-1 assembly-level tolerances come from part tolerances • Type-2 assembly-level tolerances can be altered by adjustments to the assembly process

  26. Assembly Design Process Nominal Design: Identify each KC Design a DFC for it Choose features to build constrained DFC Check for proper constraint Check for KC conflict Find a suitable assembly sequence Variational Design: Check for robustness of DFC against variations Check achievement of each KC using tolerance analysis See if a different assembly sequence gives better variation

  27. Assembly Course Topics • Assembly in the small: • Physics of part mating • Assembly in the large: • Key characteristics • Constraint • Tolerances • DFA • Product architecture, customization • A class project on these topics lasts all term

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